Obsessive-compulsive disorder (OCD), once thought to be rare, has recently been estimated to have a lifetime prevalence of 2 to 3%. Thus, more than 4 million people in the United States suffer from this potentially debilitating disorder. OCD is characterized by persistent and senseless ideas, thoughts, impulses or images that are experienced as intrusive (obsessions), and repetitive, purposeful and intentional behaviors that are often performed in response to an obsession (compulsions). The overall aim of this project is to investigate the familial aggregation of OCD and to examine the nature of the genetic factors contributing to its etiology. Specifically, we propose to seek out specific monogenic mechanisms that predispose individuals to OCD; to study further the coexistence of specific monogenic mechanisms that predispose individuals to OCD; to study further the coexistence of other mental disorders in patients affected with OCD and the incidence of OCD and other psychiatric diseases in the first and second degree relatives of these patients in order to study ethnic and gender differences in the etiology and symptomatology of OCD. We will also investigate the effects of gender on symptom presentation. Patients and their families are currently being ascertained through the Tulane Medical Center OCD clinic in New Orleans, Louisiana and through the clinics at the Instituto Mexicano de Psiquiatr!a in Mexico City, Mexico. Eventually, linkage analysis will be done on families obtained though the Mexican clinics to seek out the chromosomal location of the gene (or genes) contributing to OCD.